1. Field of the Invention
The present invention relates to an authentication system for performing authentication of a mounted device, and an authentication code convertor for converting an authentication code from a mounted device into an authentication code of an authentication device for performing authentication of the mounted device.
2. Description of the Related Art
In multi-function printers (hereinafter referred to as “printers”) having a print function and a copy function, consumables and regular replacement parts are often replaced unit by unit. For example, a fixing device essential to the printer, which is used in a high-temperature environment, has a shorter life than a printer main body, and is therefore often designed so as to facilitate the replacement.
Further, many printers have expanded functions so that an optional feeder and an optional sheet delivery device can be mounted to the printer. However, when a unit other than an apparatus compatible with the printer main body (printer specifications) (for example, a unit for a printer main body of another kind) is erroneously mounted to the printer, there is a fear not only that the printer may fail to operate but also that a failure may occur in the printer main body. Therefore, particularly with regard to a unit such as a fixing device for which high reliability is demanded, it is important to determine whether or not the unit compatible with the printer main body is mounted. When it is determined that the unit incompatible with a main body is mounted, it is necessary to call a user's attention. An authentication technology, which is a kind of encryption technology, is already used to determine whether or not the mounted apparatus is the compatible unit. As examples of authentication methods to which the authentication technology is applied include a challenge-response authentication method that uses common key encryption and a digital signature that uses public key encryption.
The challenge-response authentication method is premised that a to-be-authenticated device serving as a mounted device and an authentication device serving as an authentication source each have a common key. The to-be-authenticated device encrypts data given by the authentication device (challenge data) and returns encrypted data (response data) to the authentication device, while the authentication device determines authentication based on whether or not the encrypted data returned from the to-be-authenticated device is correct. The encryption is performed here by, for example, a message authentication code technology described in “ISO/IEC 9797-1:1999, Information technology-Security techniques-Message Authentication Codes (MACs)—Part 1: Mechanisms using a block cipher.” Further, by using the message authentication code technology, it is possible to simultaneously send internal data of the to-be-authenticated device and authenticate the to-be-authenticated device.
In the challenge-response authentication method, which executed based on the common key encryption, an authentication result is determined based on whether or not the authentication device and the to-be-authenticated device have the same encryption key (common key), and hence it is important that the common key in use is concealed from outside. Therefore, in order to prevent common key information from being leaked with ease, it is desired that common key data and an authentication operation be handled by a tamper-resistant chip having high security.
On the other hand, such a digital signature as described in “FIPS PUB 186-3: Federal Information Processing Standards Publication Digital Signature Standard (DSS)” is premised that the to-be-authenticated device serving as a mounted device and the authentication device serving as an authentication source have a pair of keys instead of the common key. In a digital signature technology, there exist two entities, in other words, a signer (to-be-authenticated device) for generating a digital signature and a verifier (authentication device) for verifying correctness of a signature. Then, the verifier uses a public key encryption technology to verify whether or not a message sent by the signer has been certainly generated by the signer. In the case of the digital signature technology, the authentication device and the to-be-authenticated device do not have the common key, and hence it suffices to conceal only a secret key included on a to-be-authenticated device side without the need to conceal a public key included on an authentication device side. Therefore, it suffices that only the key data and the authentication operation on the to-be-authenticated device side are handled by the tamper-resistant chip.
However, the tamper-resistant chip equipped with the public key encryption technology is more expensive than the tamper-resistant chip equipped with a common key encryption technology. Therefore, in order to reduce costs, it is desired that the tamper-resistant chip equipped with the common key encryption technology be mounted to the consumables and the regular replacement parts serving as the to-be-authenticated device.
An authentication system for ensuring that the compatible to-be-authenticated device is supplied is proposed in, for example, Japanese Patent Application Laid-Open No. 2000-76063. In the authentication system disclosed in Japanese Patent Application Laid-Open No. 2000-76063, the authentication device reads data inside the to-be-authenticated device and signature data of data obtained by encrypting a digest of the data, and compares the digest of the read data with the decrypted signature data, to thereby authenticate the to-be-authenticated device. The to-be-authenticated device combines the challenge data supplied by the authentication device, the digest stored in the to-be-authenticated device, and common secret data shared with the authentication device, again generates a digest thereof, and returns the digest as response data. The authentication device authenticates the to-be-authenticated device by determining whether or not the response data is identical with a digest of data obtained by combining the challenge data, the decrypted signature data, and the common secret data shared with the authentication device.
However, in the above-mentioned related art, in the case where the consumables and the regular replacement parts are mounted, the authentication is not performed by the printer itself but needs to be performed by an external apparatus, such as a personal computer, connected to the printer main body. Further, in a case where the common key encryption is used as the authentication technology, the tamper-resistant chip needs to be mounted to both the authentication device and the to-be-authenticated device in order to satisfy security. However, there is a problem in that it is difficult to mount the tamper-resistant chip, which is a custom part, to a general-purpose personal computer and that the personal computer cannot be used as an authentication apparatus. Further, in a case where the public key encryption is used as the authentication technology, there is no need to mount the tamper-resistant chip to the personal computer serving as the authentication apparatus. However, the tamper-resistant chip equipped with the public key encryption needs to be mounted to the to-be-authenticated device serving as the consumable, which raises a problem of an increase in the costs.